The heart as an electrical pump Flashcards
What are the 6 main components of the cardiac conduction system?
Sinoatrial node (SAN)
Internodal tracts
Atrioventricular node (AVN)
Bundle of His
Bundle branches (left & right)
Purkinje fibres
Describe the location of the SAN.
Posterior and upper wall of the right atrium, close to the opening of the SVC
The SAN has intrinsic automaticity – what does this mean?
It can spontaneously generate action potentials
What is the frequency of spontaneous action potentials generated by the SAN?
Once every second
Why can’t electrical signal propagate from the atrium to the ventricles directly?
Because of the fibrous cardiac skeleton – it electrically isolates the atria from the ventricles
Why is it important that electrical signal is transmitted via the AVN as opposed to directly from the atria to the ventricles?
The AV node introduces a physiological delay of approximately 100ms in conduction, allowing time for the atria to contract and complete ventricular filling before the ventricles contract
List the conduction velocities of the atria, AV node, His-Purkinje, ventricle
Atria – 1 m/s
AV node – 0.01 – 0.05 m/s
His-Purkinje – 2-4 m/s
Ventricle – 1 m/s
List the 3 latent pacemakers of the heart and their discharge rates.
AV node – 40-50 bpm
Bundle of His – 40-50 bpm
Purkinje fibres – 20 bpm
Ectopic pacemakers can arise from what?
Hyperexcitability of atrial and ventricular myocytes
Describe the pattern of contraction in the ventricles.
Contraction happens from the apex to the base
What is the resting membrane potential of myocytes?
-65 to -90 millivolts (mV)
What are the 2 types of cardiac action potentials?
Slow response potentials
Fast response potentials
Where are slow response potentials found?
SAN and AVN
Describe the speed of depolarization in slow response potentials and what this is due to.
Slowly depolarizing cells due to unstable resting membrane potential (‘pacemaker’ potential)
Describe the speed of depolarization in slow response potentials and what this is due to.
Rapidly depolarizing cells due to stable resting membrane potential
Cardiac action potentials have how many phases?
5 phases – Phase 0 – Phase 4
What is Phase 0 of the cardiac action potential?
Depolarisation
Cardiac action potentials are divided into phases according to what?
Ion movements
What is Phase 1 of the cardiac action potential?
Rapid repolarization
What is Phase 2 of the cardiac action potential?
Plateau
What is Phase 3 of the cardiac action potential?
Repolarization
What is Phase 4 of the cardiac action potential?
Resting
What phases do slow response cardiac action potentials have?
Phases 0, 3, 4
What phases do fast response cardiac action potentials have?
Phases 0, 1, 2, 3, 4
Describe what happens during each phase of slow response action potentials.
Phase 0 – Ca2+ influx causes slow depolarization
Phase 3 – K+ efflux causes repolarization
Phase 4 – membrane is permeable to Na+ and K+. Net inward current causes slow depolarization
Which current gives the pacemaker cells their intrinsic automaticity?
Funny current
In terms of the parasympathetic nervous system – which nerve innervates the heart, which specific areas does it innervate, which neurotransmitter, which receptors, and what effect does it have?
Vagus nerve – mostly innervates SAN and AVN
Acetylcholine binds to muscarinic M2-cholinergic receptors, slowing the heart rate
In terms of the sympathetic nervous system – which specific area of the heart does it innervate, which mediator, which receptors, and what effect does it have?
Innervates conduction system and myocardium
Noradrenaline binds to B1-adrenoreceptors, increasing the heart rate
Describe the 3 ways in which the parasympathetic nervous system slows heart rate.
ACh reduces the influx of calcium ions, which leads to a less steep Phase 4, meaning more time is required to reach the threshold
ACh causes K+ channels to open, increasing K+ efflux which leads to hyperpolarization, so more time is required to reach the threshold
Calcium channels are modulated – the threshold for activation is increased.
Define negative chronotropy and where it occurs.
A decrease in heart rate – a slowing of the frequency at which the heart beats
Occurs in the SAN
Define negative dromotropy and where it occurs.
A decrease in the speed of electrical conduction through the heart
Occurs in the AVN
What is the overall effect of the parasympathetic nervous system on the SAN?
Decreases pacemaker firing
What is the overall effect of the parasympathetic nervous system on the AVN?
Decreases conduction velocity
Describe the 2 ways in which the sympathetic nervous system increases heart rate.
Noradrenaline increases the influx of calcium, which leads to a steeper Phase 4, causing faster depolarization
Calcium channels are modulated – the threshold for activation is decreased
What is the overall effect of the sympathetic nervous system on the SAN?
Increases pacemaker firing
What is the overall effect of the sympathetic nervous system on the AVN?
Increases conduction velocity
Where are fast response potentials found?
Atrial + ventricular myocytes
Purkinje fibres
Describe what happens during each phase of fast response action potentials.
Phase 0 – Rapid influx of Na+ causes rapid depolarization
Phase 1 – initial K+ efflux, causing rapid and transient repolarization
Phase 2 – influx of Ca2+ balances K+ efflux, creating a sustained plateau
Phase 3 – K+ efflux causes repolarization
Phase 4 – stable resting membrane potential
What are the 2 refractory periods in a fast response potential?
Absolute refractory period
Relative refractory period
What cannot happen during the absolute refractory period?
No new action potential can be generated
When does the absolute refractory period occur?
From the beginning of Phase 0 through to Phase 3
When does the relative refractory period occur?
From Phase 3 through to the early part of Phase 4
Describe the generation of action potentials in the relative refractory period.
Action potentials can be generated but a stronger than normal stimulus is needed
Give 2 reasons why the refractory period in fast response potentials is important.
Ensures unidirectional propagation of the action potential
Ensures adequate time for ventricular filling prior to subsequent contraction
What is the outer membrane of cardiomyocytes called?
Sarcolemma
What are the thick filaments of sarcomeres made of?
Myosin
What are the thin filaments of sarcomeres made of?
Actin, tropomyosin, and troponin
In cardiomyocytes, what are transverse tubules and what is their function?
Invaginations of the sarcolemma that extend deep into the cardiomyocyte
They allow the depolarization of the membrane to penetrate muscle fibre
What is the sarcoplasmic reticulum?
Intracellular organelle which stores calcium
Cardiomyocytes connect to each other via what?
Intercalated discs
Which 2 specialized structures are present in cardiomyocytes which provide mechanical support and electrical coupling?
Desmosomes and gap junctions
Which proteins are gap junctions composed of?
Connexins
What is excitation-contraction coupling?
The mechanism that translates a cardiac action potential into muscle contraction
Describe what happens during excitation-contraction coupling.
T tubule membrane depolarisation. This causes the opening of voltage-gated calcium channels. A small influx of calcium binds to and activates ryanodine receptors on the sarcoplasmic reticulum, causing a large release of calcium. Calcium binds to troponin C, causing contraction. Calcium removal from the cytoplasm leads to relaxation – it is actively pumped back into the SR by SR Ca2+-ATPase (SERCA) and removed from the cell via NCX
What is the process called where calcium binding to ryanodine receptors on the SR releases calcium?
Calcium-induced calcium release
